Database structur for a messaging function in realm io

I currently developing an app with a messaging function.
Private, group and global messaging should be Implemented.

I need some help to create a database structur for this feature.

Will this work? Maybe there is a better solution available? Do I still need a ID for the Messages?(User and Groups already have a ID):

public class User {
//...
public RealmList"<"PrivateMessage">" msgs = null;
}

public class PrivateMessage {
private String Message = "";
private long Time = 0;
//getter setter
}

public class Group {
//...
public RealmList"<"GroupMessage">" msgs = null;
}

public class GroupMessage {
private String Sender = "";
private String Message = "";
private long Time = 0;
//getter setter
}